The major work in our laboratory has been in studies on the regulation of dihydrofolate reductase gene expression in human breast cancer cells. Human DHFR genomic sequences have been cloned from a gene amplified drug resistant cell line and have been used to form a DHFR minigene. The regulation of this minigene has been studied in a cellular expression system following transfection into mutant Chinese hamster cells. We have also developed retroviral vectors which contain an altered DHFR gene which can efficiently transfer methotrexate resistance to a wide variety of cells. Finally, we are also studying the mechanisms responsible for the development of pleiotropic drug resistance in an MCF-7 cell line isolated in our lab. These studies have identified increased levels of proteins and amplified DNA sequences which may play a role in the development of resistance.